Transducer of regulated CREB and late phase long-term synaptic potentiation Hao Wu, Yang Zhou and Zhi-Qi Xiong Institute of Neuroscience and Key Laboratory of Neurobiology, Shanghai Inst
Trang 1Transducer of regulated CREB and late phase long-term synaptic potentiation
Hao Wu, Yang Zhou and Zhi-Qi Xiong
Institute of Neuroscience and Key Laboratory of Neurobiology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China
Introduction
Synaptic plasticity, the change in the strength of
neur-onal connections in the brain, is thought to underlie
learning and memory [1–3], and may play a crucial role
in the pathogenesis of a variety of neurological
disor-ders, including drug addiction [4] One form of synaptic
plasticity that has received much attention is long-term
potentiation (LTP), an activity-dependent long-lasting
increase of synaptic strength [5] Like memory, LTP
can be divided into two distinct phases: an early phase
(E-LTP) which lasts only minutes to few hours and
involves modification of preexisting proteins; and a late
phase (L-LTP), which persists from hours to days and
requires gene transcription and protein synthesis [6]
Despite the fact that LTP was discovered by Bliss
et al [7] more than three decades ago, the molecular
and cellular mechanisms underlying this phenomenon are still not well understood One major advance in this effort occurred when the properties of
N-methyl-d-aspartate-type glutamate receptors (NMADR) were first elucidated in the mid-1980s, and at about the same time, researchers found that N-methyl-d-aspar-tate receptor (NMDAR) antagonists prevented LTP NMDAR act as detectors of the coincidence between the depolarization of postsynaptic membrane and the presence of glutamate in the synaptic cleft The resulting Ca2+ transients result in LTP [8–10] A likely molecular cascade is that Ca2+ influx through NMDAR activates one or more protein kinases in the postsynaptic neuron such as Ca2+⁄ calmodulin-depend-ent protein kinases II and IV, protein kinase (PK)A, PKC, and mitogen-activated protein kinase, etc [11] Activation of these kinases induces gene expression
Keywords
CREB; hippocampus; LTP; TORCs
Correspondence
Z.-Q Xiong, Laboratory of Neurobiology of
Disease, Institute of Neuroscience, Chinese
Academy of Sciences, 320 Yue Yang Road,
Shanghai 200031, China
Fax: +86 21 5492 1735
Tel: +86 21 5492 1716
E-mail: xiongzhiqi@ion.ac.cn
(Received 28 January 2007, revised 29 April
2007, accepted 8 May 2007)
doi:10.1111/j.1742-4658.2007.05891.x
In the central nervous system, long-term adaptive responses to changes in the environment, such as the processes involved in learning and memory, require the conversion of extracellular stimuli into intracellular signals Many of these signals involve the induction of gene expression The late, transcription- and translation-dependent phase of long-term synaptic potentiation (L-LTP) is an attractive cellular model for long-lasting mem-ory formation The transcription factor cAMP response element-binding protein (CREB) plays an essential role in the maintenance of L-LTP How-ever, how synaptic signals propagate to the nucleus to initiate CREB-target gene expression is unclear Recent studies indicate that the CREB transdu-cer of regulated CREB activity 1 coactivator undergoes neuronal activity-dependent translocation from the cytoplasm to the nucleus, a process required for CRE-dependent gene expression and the maintenance of L-LTP in the hippocampus
Abbreviations
BDNF, brain derived neurotrophic factor; bZIP, basic leucine zipper; CRE, cAMP response element; CREB, CRE-binding protein; CBP, CREB-binding protein; DN, dominant-negative; KID, kinase inducible domain; LTP, long-term potentiation; NMDAR, N-methyl- D -aspartate receptor;
PK, protein kinase; SIK, salt inducible kinase; VGCC, voltage-gated calcium channel.
Trang 2and synthesis of new proteins, a process required for
cAMP response element (CRE)-dependent gene
expres-sion and L-LTP
Role of CREB-target gene expression
in L-LTP
Pharmacological evidence demonstrated that the
expression of L-LTP in hippocampus requires both
gene transcription and protein synthesis [12–14]
Fur-ther study indicated that the induction of L-LTP
corre-lates with the expression of CRE-dependent gene
expression [15] Although some studies argued against
the role of CREB in hippocampal L-LTP and memory
formation [16,17], accumulating evidence from both
invertebrates and vertebrates has demonstrated the
essential role of CREB in mediating hippocampal
L-LTP and memory process [15,18–21] Overexpressing
a constitutively active form of CREB (VP16-CREB)
facilitates hippocampal L-LTP induction probably via
increased BDNF (brain derived neurotrophic factor)
expression [22,23] One well established mechanism for
CREB-mediated gene transcription is that upon being
phosphorylated at Ser133, CREB undergoes
conforma-tional change and recruits CREB binding protein
(CBP) and other elements to initiate target gene
tran-scription [3,24]
Although studies have demonstrated the importance
of CREB phosphorylation, in particular at Ser133, for
CRE-driven gene transcription [3,25], stimuli which
induce Ser133 phosphorylation do not completely
par-allel CREB dependent transcription [15] Some
extra-cellular stimuli are capable of phosphorylating CREB
at Ser133 but fail to trigger CREB-target gene
tran-scription [26–29] Moreover, the inconsistent kinetics
between CREB Ser133 phosphorylation and
CREB-dependent gene transcription has also been reported
That is, although persistent phosphorylation was
observed following membrane-depolarizing stimulation
in primary cortical neurons, an in vitro nuclear run-on
assay showed that CREB-dependent gene transcription
only occurs in a short time window, implicating the
existence of a switch-off mechanism in controlling
the kinetics of gene expression other than Ser133
phosphorylation [30].These findings suggest at least
one additional factor is involved in the regulating
CRE-target gene transcription
Transducers of regulated CREB activity
(TORCs)
With respect to the structure–function relationship of
CREB activity, it was shown that deletion of the basic
lucine zipper (bZIP) domain of CREB remarkably inhibited CRE-target gene expression [29,31], suggest-ing that a modulatory mechanism works via this domain Indeed, phylogenic analysis of the cDNAs of CREB gene from Caenorhabditis elegans to mammals indicates that the primary amino acid sequence of CREB is highly conserved in at least two domains, namely kinase inducible domain (KID) and bZIP DNA binding⁄ dimmerization domain [32] KID in CREB, encompassing Ser133 site, binds to CBP in a phosphorylation-dependent manner [33] Studies from CBP mutant mice showed that CBP is critical for the late-phase of hippocampal LTP and some forms of long-term memory [34]
Efforts to identify novel CREB coactivators through bZIP domain led to the discovery of a conserved fam-ily of coactivators: TORCs TORC famfam-ily proteins are capable of binding with the bZIP domain independent
of phosphorylation status of CREB at Ser133, and to specifically potentiate CRE-mediated reporter gene transcription [35,36] In the mammalian genome, the TORC family consists of three members, TORC1, TORC2 and TORC3 [35,36] Its Drosophila homolog dTORC was identified via database searching [36] and has been shown to function similar to its mammalian counterparts [37] Whereas there are no extensive homologies among three mammalian TORCs, a highly conserved N-terminal coiled coil domain can be mapped to each member and this domain is respon-sible for tetramer formation and for CREB activity potentiation [35] Recently, it was found that TORC2
is a key regulator of fasting glucose metabolism, thereby shedding light on a long-standing puzzle in which insulin and glucagon can equally induce canon-ical CREB phosphorylation, but have opposite effects
on CREB-target gene transcription and glucose meta-bolism [38,39] Most recently, it has also been reported that TORCs are critical for the mitochondrial biogen-esis in muscle cells [40]
Gene profiling analyses showed that mRNA levels
of three TORCs are differentially expressed in distinct tissues [35] To gain insight into the potential function
of TORCs in the central nervous system, we cloned the TORC isoforms from the adult rat brain and found both TORC1 mRNA and protein are abundant
in the hippocampus [41] Consistent with earlier studies with TORC2 which translocates into nucleus in response to elevated intracellular cAMP and⁄ or cal-cium [42], nuclear accumulation of TORC1 could be induced by increasing intracellular cAMP level, Ca2+ influx via voltage-gated calcium channel (VGCC)
or activation of NMDAR in primary hippocampal neurons [41]
Trang 3Regulation of CREB-dependent gene
transcription and hippocampal L-LTP
by TORC1
The nuclear translocation property of TORC1 makes
it an attractive candidate in relaying signals from
syn-apse to nucleus elicited by neuronal activity [43] We
used CRE-reporter gene assay to examine the
func-tional consequence of neuronal activity-dependent
TORC1 nuclear accumulation Overexpressing a
dom-inant-negative (DN) TORC1 or knockdown of
endo-genous TORC1 inhibits neuronal activity-dependent
expression of CRE-reporter gene; whereas
overexpress-ing the wild-type (WT) TORC1 increases both basal
and neuronal activity-induced CRE-reporter gene
expression The expression of endogenous BDNF, a well-known CREB target gene [30] implicated in the synaptic plasticity [44], is also up-regulated by TORC1 [41]
Since CRE-target gene expression is critical for the maintenance of L-LTP [23], we thus tested the func-tional role of TORC1 in L-LTP in the Shaffer collat-eral pathway of rat hippocampal slices This pathway
is derived from axons that project from the CA3 region to the CA1 region and is utilized extensively to study NMDA receptor-dependent LTP E-LTP in this pathway can be induced by one train of high frequency stimulation and lasts approximately 1 h; L-LTP can be induced by three or four trains of high frequency sti-mulation and lasts more than 3 h Using this model,
C
Fig 1 Activity-dependent nuclear translocation of TORC1 contributes to L-LTP maintenance (A) Subcellular distribution of TORC1 in CA1 neurons after basal stimulation (Basal), E-LTP induction (one train of high frequency stimuation, 1 · HFS) and L-LTP induction (four trains of high frequency stimulation, 4 · HFS) Distribution of TORC1 was examined by immunohistochemical staining L-LTP induction induces nuc-lear and perinucnuc-lear accumulation of TORC1 in CA1 neurons (B) DN-TORC1 infection blocks L-LTP maintenance in hippocampal slices Induction of L-LTP was marked with four arrowheads Maintenance of L-LTP was evaluated by comparing the field excitatory postsynaptic potential (fEPSP) slope before L-LTP induction (as indicated at the zero point of x-axis by ‘1’) with the fEPSP slope 180 min after L-LTP induction (as indicated by ‘2’) Typical traces of fEPSP at time point ‘1’ and ‘2’ are shown in the upper panel (C) WT-TORC1 infection low-ered the threshold for L-LTP induction in hippocampal slices Induction of E-LTP is marked by an arrowhead fEPSP of time point ‘1’ and ‘2’ was compared for evaluation of E-LTP or L-LTP.
Trang 4we found that the induction of L-LTP, but not E-LTP,
triggers robust nuclear and perinuclear accumulation
of TORC1 in the CA1 neurons of hippocampal slices
(Fig 1A) Studies of the phosphorylation level of
CREB after the same stimulation protocols revealed
that remarkable phospho-CREB was induced only
after E-LTP but not after L-LTP Thus, nuclear
accu-mulation of TORC1, but not CREB phosphorylation,
correlates with L-LTP induction in hippocampal slices
[15,41] We further found that overexpressing the
DN-TORC1 suppressed the maintenance of L-LTP
without affecting E-LTP, whereas overexpressing the
wild-type form of TORC1 facilitated the induction of
L-LTP (Fig 1B) Most recently, another independent
study also revealed TORC1 is required for the
syner-gistic activation of CREB-mediated transcription by
Ca2+ and cAMP and the maintenance of L-LTP [45]
In this work, Kovacs et al [45] generated a membrane
permeable peptide of dominant-negative TORC1 They
found that acute delivery of TORC1
dominant-negat-ive peptide into rat hippocampal slices blocked the
maintenance of L-LTP induced by three trains of high
frequency stimulation Taken together, these findings
indicate that TORC1 acts as the coincidence detector
for sensing intracellular Ca2+ and cAMP changes
induced by neuronal activity and is translocated to
nucleus to drive CREB-target gene transcription and
maintain L-LTP (Fig 2)
Perspectives
In the central nervous system, activity-regulated
CREB-target gene transcription has been implicated in
diverse processes, ranging from neuronal development
and synaptic plasticity to disease conditions [3] It
would be interesting to investigate whether and to
what extent TORC1 participates in these processes If
so, subsequent efforts to reveal the dynamic regulation
of TORC1 activity should have therapeutic
impli-cations for a lot of neurological disorders Since the
subcellular distribution of TORCs is dependent on its
phosphorylation status [42], an interesting question is
what types of kinase and⁄ or phosphatase are
respon-sible for this shuttling process of TORC1 in neurons
In cell line, salt inducible kinase (SIK) and protein
phosphatase calcineurin regulate the phosphorylation
status of TORC2 [42] Preliminary results showed that
SIK mRNA could be readily detected from the
hippo-campus (Y.-F Li & Z.-Q Xiong, unpublished data)
and calcineurin was also found to be enriched in
neu-rons [46] Thus, it is most likely that SIKs and
cal-cineurin may be the primary candidates regulating its
phosphorylation status in response to neuronal
activ-ity However, the involvement of other kinases or phophatases in regulation of TORC1 activity is also possible Efforts to identify these kinases⁄ phosphatases will provide more insight into the regulation of TORC function in the nervous system
Earlier studies reported that CREB target genes including c-fos, BDNF and Nur⁄ 77 are transcribed only in a transient manner, whereas CREB phosphory-lation at Ser133 persists for more than 6 h [30], sug-gesting the existence of additional molecule element regulate the kinetics of CREB-target gene transcription
A
B
Fig 2 Neuronal signaling promotes nuclear accumulation of TORC1 and BDNF expression and maintenance of L-LTP (A) The schematic drawing shows that Ca2+influx via VGCC or NMDAR or increased level of intracellular cAMP can promote nuclear accumu-lation of TORC1, and nuclear TORC1 acts as a CREB coactivator to potentiate the expression of CREB target gene BDNF in neurons (B) The schematic drawing shows that TORC1 nuclear accumula-tion activates transcripaccumula-tion of CRE-target genes in neurons, thus leads to potential of synaptic transmission.
Trang 5bypass CREB phosphorylation in neurons
Interest-ingly, detailed analysis about dynamics of the nuclear
translocation of TORC1 showed that nuclear
accumu-lation of TORC1 peaks at 1 h and returns to basal
level approximately 6 h following member-depolarizing
stimulation in cortical neurons (Y.-F Li & Z.-Q
Xiong, unpublished data), which correlates well with
the transcription kinetics of the CREB target gene
Further work to delineate the contribution of CREB
phosphorylation versus nuclear translocation of
TORC1 to the transcription kinetics of CREB-target
gene expression will help our understanding the
regula-tory mechanisms of neuronal activity-dependent
CRE-target gene expression and the role in neuronal
devel-opment and synaptic plasticity
Acknowledgements
We thank Ms Ye-Fei Li for her help with the artwork
The authors’ work is supported by the National Basic
Research Program of China Grant (2006CB806600),
the Key State Research Program of China Grant
(2006CB943900), national ‘863’ high-tech research and
development program (2006AA02Z166), and ‘Hundred
Talents Plan’ of the Chinese Academy of Sciences and
Shanghai Pujiang Program Grant (05PJ14114)
References
1 Whitlock JR, Heynen AJ, Shuler MG & Bear MF
(2006) Learning induces long-term potentiation in the
hippocampus Science 313, 1093–1097
2 Pastalkova E, Serrano P, Pinkhasova D, Wallace E,
Fenton AA & Sacktor TC (2006) Storage of spatial
information by the maintenance mechanism of LTP
Science 313, 1141–1144
3 Lonze BE & Ginty DD (2002) Function and regulation
of CREB family transcription factors in the nervous
sys-tem Neuron 35, 605–623
4 Chao J & Nestler EJ (2004) Molecular neurobiology of
drug addiction Annu Rev Med 55, 113–132
5 Lynch MA (2004) Long-term potentiation and memory
Physiol Rev 84, 87–136
6 Kandel ER (2001) The molecular biology of memory
storage: a dialogue between genes and synapses Science
294, 1030–1038
7 Bliss TV & Lomo T (1973) Long-lasting potentiation of
synaptic transmission in the dentate area of the
anaes-thetized rabbit following stimulation of the perforant
path J Physiol 232, 331–356
8 Malenka RC, Kauer JA, Zucker RS & Nicoll RA
(1988) Postsynaptic calcium is sufficient for potentiation
of hippocampal synaptic transmission Science 242,
81–84
9 MacDermott AB, Mayer ML, Westbrook GL, Smith SJ
& Barker JL (1986) NMDA-receptor activation increa-ses cytoplasmic calcium concentration in cultured spinal cord neurones Nature 321, 519–522
10 Lynch G, Larson J, Kelso S, Barrionuevo G & Schottler
F (1983) Intracellular injections of EGTA block induc-tion of hippocampal long-term potentiainduc-tion Nature 305, 719–721
11 Miyamoto E (2006) Molecular mechanism of neuronal plasticity: induction and maintenance of long-term potentiation in the hippocampus J Pharmacol Sci 100, 433–442
12 Scharf MT, Woo NH, Lattal KM, Young JZ, Nguyen
PV & Abel T (2002) Protein synthesis is required for the enhancement of long-term potentiation and long-term memory by spaced training J Neurophysiol 87, 2770– 2777
13 Frey U, Frey S, Schollmeier F & Krug M (1996) Influence of actinomycin D, a RNA synthesis inhi-bitor, on long-term potentiation in rat hippo-campal neurons in vivo and in vitro, J Physiol 490, 703–711
14 Nguyen PV, Abel T & Kandel ER (1994) Requirement
of a critical period of transcription for induction of a late phase of LTP Science 265, 1104–1107
15 Impey S, Mark M, Villacres EC, Poser S, Chavkin C
& Storm DR (1996) Induction of CRE-mediated gene expression by stimuli that generate long-lasting LTP
in area CA1 of the hippocampus Neuron 16, 973–982
16 Balschun D, Wolfer DP, Gass P, Mantamadiotis T, Welzl H, Schutz G, Frey JU & Lipp HP (2003) Does cAMP response element-binding protein have a pivotal role in hippocampal synaptic plasticity and hippocampus-dependent memory? J Neurosci 23, 6304–6314
17 Pittenger C, Huang YY, Paletzki RF, Bourtchouladze
R, Scanlin H, Vronskaya S & Kandel ER (2002) Reversible inhibition of CREB⁄ ATF transcription factors in region CA1 of the dorsal hippocampus disrupts hippocampus-dependent spatial memory Neuron 34, 447–462
18 Bourtchuladze R, Frenguelli B, Blendy J, Cioffi D, Schutz G & Silva AJ (1994) Deficient long-term memory
in mice with a targeted mutation of the cAMP-responsive element-binding protein Cell 79, 59–68
19 Yin JC, Wallach JS, Del Vecchio M, Wilder EL, Zhou H, Quinn WG & Tully T (1994) Induction of a dominant negative CREB transgene specifically blocks long-term memory in Drosophila Cell 79, 49–58
20 Frank DA & Greenberg ME (1994) CREB: a mediator
of long-term memory from mollusks to mammals Cell
79, 5–8
21 Silva AJ, Kogan JH, Frankland PW & Kida S (1998) CREB and memory Annu Rev Neurosci 21, 127–148
Trang 622 Barco A, Alarcon JM & Kandel ER (2002) Expression
of constitutively active CREB protein facilitates the late
phase of long-term potentiation by enhancing synaptic
capture Cell 108, 689–703
23 Barco A, Patterson S, Alarcon JM, Gromova P,
Mata-Roig M, Morozov A & Kandel ER (2005) Gene
expression profiling of facilitated L-LTP in
VP16-CREB mice reveals that BDNF is critical for the
maintenance of LTP and its synaptic capture Neuron
48, 123–137
24 Johannessen M, Delghandi MP & Moens U (2004)
What turns CREB on? Cell Signal 16, 1211–1227
25 Kornhauser JM, Cowan CW, Shaywitz AJ, Dolmetsch
RE, Griffith EC, Hu LS, Haddad C, Xia Z &
Green-berg ME (2002) CREB transcriptional activity in
neurons is regulated by multiple, calcium-specific
phos-phorylation events Neuron 34, 221–233
26 Liu FC & Graybiel AM (1996) Spatiotemporal
dynam-ics of CREB phosphorylation: transient versus sustained
phosphorylation in the developing striatum Neuron 17,
1133–1144
27 Bito H, Deisseroth K & Tsien RW (1996) CREB
phos-phorylation and dephosphos-phorylation: a Ca(2+)- and
sti-mulus duration-dependent switch for hippocampal gene
expression Cell 87, 1203–1214
28 Thompson MA, Ginty DD, Bonni A & Greenberg ME
(1995) L-type voltage-sensitive Ca2+channel activation
regulates c-fos transcription at multiple levels J Biol
Chem 270, 4224–4235
29 Bonni A, Ginty DD, Dudek H & Greenberg ME
(1995) Serine 133-phosphorylated CREB induces
transcription via a cooperative mechanism that may
confer specificity to neurotrophin signals Mol Cell
Neurosci 6, 168–183
30 Tao X, Finkbeiner S, Arnold DB, Shaywitz AJ &
Greenberg ME (1998) Ca2+influx regulates BDNF
transcription by a CREB family transcription
factor-dependent mechanism Neuron 20, 709–726
31 Sheng M, Thompson MA & Greenberg ME (1991)
CREB: a Ca(2+)-regulated transcription factor
phos-phorylated by calmodulin-dependent kinases Science
252, 1427–1430
32 Bito H & Takemoto-Kimura S (2003) Ca(2+)⁄ CREB ⁄
CBP-dependent gene regulation: a shared mechanism
critical in long-term synaptic plasticity and neuronal
survival Cell Calcium 34, 425–430
33 Chrivia JC, Kwok RP, Lamb N, Hagiwara M,
Mont-miny MR & Goodman RH (1993) Phosphorylated
CREB binds specifically to the nuclear protein CBP
Nature 365, 855–859
34 Alarcon JM, Malleret G, Touzani K, Vronskaya S, Ishii
S, Kandel ER & Barco A (2004) Chromatin acetylation,
memory, and LTP are impaired in CBP+⁄ – mice: a
model for the cognitive deficit in Rubinstein–Taybi
syn-drome and its amelioration Neuron 42, 947–959
35 Conkright MD, Canettieri G, Screaton R, Guzman E, Miraglia L, Hogenesch JB & Montminy M (2003) TORCs: transducers of regulated CREB activity Mol Cell 12, 413–423
36 Iourgenko V, Zhang W, Mickanin C, Daly I, Jiang C, Hexham JM, Orth AP, Miraglia L, Meltzer J, Garza D
et al.(2003) Identification of a family of cAMP response element-binding protein coactivators by gen-ome-scale functional analysis in mammalian cells Proc Natl Acad Sci USA 100, 12147–12152
37 Bittinger MA, McWhinnie E, Meltzer J, Iourgenko V, Latario B, Liu X, Chen CH, Song C, Garza D & Labow M (2004) Activation of cAMP response element-mediated gene expression by regulated nuclear transport
of TORC proteins Curr Biol 14, 2156–2161
38 Canettieri G, Koo SH, Berdeaux R, Heredia J, Hedrick
S, Zhang X & Montminy M (2005) Dual role of the coactivator TORC2 in modulating hepatic glucose output and insulin signaling Cell Metab 2, 331–338
39 Koo SH, Flechner L, Qi L, Zhang X, Screaton RA, Jeffries S, Hedrick S, Xu W, Boussouar F, Brindle P
et al.(2005) The CREB coactivator TORC2 is a key regulator of fasting glucose metabolism Nature 437, 1109–1111
40 Wu Z, Huang X, Feng Y, Handschin C, Feng Y, Gul-licksen PS, Bare O, Labow M, Spiegelman B & Steven-son SC (2006) Transducer of regulated CREB-binding proteins (TORCs) induce PGC-1alpha transcription and mitochondrial biogenesis in muscle cells Proc Natl Acad Sci USA 103, 14379–14384
41 Zhou Y, Wu H, Li S, Chen Q, Cheng XW, Zheng J, Takemori H & Xiong ZQ (2006) Requirement of TORC1 for late-phase long-term potentiation in the hippocampus PLoS ONE 1, E16
42 Screaton RA, Conkright MD, Katoh Y, Best JL, Canettieri G, Jeffries S, Guzman E, Niessen S, Yates JR III, Takemori H et al (2004) The CREB coactivator TORC2 functions as a calcium- and cAMP-sensitive coincidence detector Cell 119, 61–74
43 Deisseroth K, Mermelstein PG, Xia H & Tsien RW (2003) Signaling from synapse to nucleus: the logic behind the mechanisms Curr Opin Neurobiol 13, 354–365
44 Lu B (2003) BDNF and activity-dependent synaptic modulation Learn Mem 10, 86–98
45 Kovacs KA, Steullet P, Steinmann M, Do KQ, Magist-retti PJ, Halfon O & Cardinaux JR (2007) TORC1 is a calcium- and cAMP-sensitive coincidence detector involved in hippocampal long-term synaptic plasticity Proc Natl Acad Sci USA 104, 4700–4705
46 Kincaid RL, Balaban CD & Billingsley ML (1987) Dif-ferential localization of calmodulin-dependent enzymes
in rat brain: evidence for selective expression of cyclic nucleotide phosphodiesterase in specific neurons Proc Natl Acad Sci USA 84, 1118–1122